ROBINS AIR FORCE BASE — About half of the Air Force’s C-130 cargo fleet remains grounded as Robins Air Force Base engineers and support teams continue to grapple with suspect upper wing barrel nuts.

Officials in the 330th Aircraft Sustainment Group at Robins issued a worldwide inspection order March 4 after finding five cracked nuts in a C-130H undergoing depot overhaul on base. The order effectively grounded the fleet, including the newest C-130Js, until they were inspected. The nuts and related bolts — 26 per aircraft — anchor a rainbow-shaped fitting that holds the wings to the center wing box on the aircraft’s main fuselage.

The 330th and the Warner Robins Air Logistics Center at Robins have worldwide engineering, supply, overhaul and fleet management responsibility for the Lockheed Martin-produced aircraft. The C-130 has been the tactical airlift backbone for the Air Force since it became operational in 1956.

More than 2,000 of them in 70 variants and five basic models have been produced, not only for U.S. forces but for many foreign countries.

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With 90 percent of about 600 Air Force aircraft inspected, 266 have been found with three or more suspect nuts.

“If they had two or less, it was considered a safe condition and they were returned to service,” explained Terrence May, the 330th sustainment group director.

Maintenance crews were required to identify and document the number of suspect nuts by aircraft tail number but not remove them.

The suspect nuts, traced to a single vendor, are readily identifiable, May said, although the total number of cracked nuts won’t be known until removals begin.

“You can’t tell if they are cracked by eyeballing them,” he said. “The cracked area would be on the underside of the nut in the threaded area.”

The goal is to replace all suspect nuts in the system — about 6,500 at the latest count.

Mission priorities and the supply of replacement nuts will determine when aircraft are restored to flight status.

“We have already shipped enough nuts to replace suspect ones for aircraft” in Iraq and Afghanistan, May said.

Other high-priority and time-sensitive missions are also receiving shipments.

Additional barrel nuts have been ordered through the Defense Logistics Agency. At least four other vendors make the high-strength steel component.

“It will be about April 10 before the first of the new orders come in,” the director said. “So it will be about six to eight weeks before supply catches up with demand.”

Why the nuts cracked is being analyzed. “We’re expecting a report by the middle of next week,” May said. “The lab analysis may show a manufacturing defect or it may indicate that a particular type of high-strength steel is not suitable for this application.”

Robins engineers are exploring alternatives, including restricted flight for aircraft with up to four suspect nuts.

The concern is the shock imposed on the wing joint at landing or when the aircraft hits a runway bump.

“Actual flight loads are not that much of a problem,” May said. “It’s the shock of landing that tends to be the issue. Of course operating on unimproved runways is also a concern.”

Another partial solution could be the substitution of engine truss mount nuts.

“The 402nd Maintenance Wing (at Robins) will manufacture some prototypes over the weekend,” May said. “If they can be used, we should be able to start manufacturing them early next week. That will shorten the recovery time.”

May offered praise for his engineering and technical staff for their ingenuity and for staffing a nonstop, telephone hot line for units around the world.

Although calls have diminished since the inspections order was issued, he expects them to pick up once the restricted flight guidelines are issued.

“At that point, there will be a lot of dialogue to understand what can be worked around,” May predicted.

“I’m hoping we can discontinue the hot line by Wednesday of next week.”